Use of Regular Rod Arrays to Model Heat Transfer From BWR Fuel Assemblies Inside Transport Casks

Abstract

The current work is a scoping study to determine which heat transfer effects are significant in the fuel/backfill gas region of spent nuclear fuel transport casks. A two-dimensional finite volume mesh that accurately models the geometry of a 7×7 Boiling Water Reactor (BWR) assembly with its channel in a square isothermal enclosure is constructed. The peak cladding temperature is determined using computational fluid dynamics (CFD) simulations for a range of enclosure temperatures, fuel heat generation rates, cladding surface emissivities, and for both nitrogen and helium backfill gases. This work quantifies both the effect of buoyancy induced gas motion in the fuel/backfill gas region and the conditions when it does not significantly affect heat transfer. Future cask design simulations that neglect gas motion will require less computational resources than ones that do not. This work also quantifies the sensitivity of the maximum cladding temperature to fuel cladding emissivity. This helps quantify the uncertainty of temperature predictions if the emissivity is not known. The current CFD technique must be experimentally benchmarked before it may be used with confidence to predict peak cladding temperatures in transport casks. This work indicates that the thermal resistance between a BWR assembly’s channel and the basket walls may be modeled analytically. This will reduce the effort required for benchmark experiments because they will not need to include the channel.